In a case where continuously cast slabs are rolled as a material in manufacturing heavy gauge steel plate, a high reduction ratio (thickness of a slab after casting/finish thickness of rolled steel plate) cannot be given. Thus, there is a problem that small holes that are casting defects (hereinafter referred to as “porosities”) remain about the centers of slabs in the thickness direction without being pressed enough to be collapsed, which causes product defects. In a case where continuous casting of slabs having large cross-sections is assumed for a high reduction ratio, low-speed casting is necessary because of the limit of machine length, which is very inefficient. Although such a method is also considered that ingots of large diameters are cast with common ingot casting but not continuous casting, the efficiency gets much worse than continuous casting.
The inventors of the present invention propose in Patent Literature 1 the following method for manufacturing heavy gauge steel plate in order to solve the above problem: under the condition that the reduction ratio r until finish rolling is 1.5 to 4.0, hot-rolling, as a material, a slab that is cast by reduction on the central part of the slab in the width direction by 3 to 15 mm with a pair of reduction rolls under the condition where the slab includes an unsolidified part while the solid-phase ratio of the central part of the slab in the thickness direction is no less than 0.8 and less than 1.0, to reduce the central porosity volume. Application of this method makes porosities in heavy gauge steel plate be considerably reduced by ¼ to ⅓ of the level of the porosities when an original slab, which is cast without reduction, is used as a material.
Even if the above-mentioned method of Patent Literature 1 is applied, there still remain considerable porosities in slabs for heavy gauge steel plate. Therefore, it must be said that the above-mentioned method of Patent Literature 1 is not sufficient for measures for the decrease of porosities in view of the request for the decrease of porosities, which is predicted to be more and more severe for the future, the tendency to consider it desirable that thinner slabs are cast at high speed and the reduction ratio in rolling is kept down, to finish steel plate, and so on.
Patent Literatures 2 and 3 describe continuous casting equipment for steel where plural pairs of rolls each of which is integrally formed in the axial direction with a large roll diameter of over 400 mm are arranged. While it is considered that reduction on slabs with plural pairs of rolls like this is extremely effective for decreasing porosities, the occurrence of the following problem is expected.
Arrangement of plural pairs of rolls with such large diameters causes bulging between rolls to occur several times when slabs whose central parts are unsolidified pass through the rolls. This brings about worse segregation of components such as carbon, sulfur and phosphorus in the central parts of the slabs (centerline segregation), occurrence of cracks on solidification interfaces (internal cracking), and so on. Even if completely solidified slabs pass through the rolls with large diameters, and reduction is tried to be carried out thereon with multistage rolls with large diameters in order to press to collapse porosities that occur in solidifying, there is a problem that reduction between the continuous rolls makes work hardening progress and the reduction does not progress so much.